Internet Protocol (IP) Multimedia Core Network Subsystem (IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (3GPP), and it constructs an open and flexible service environment to support multimedia applications and provide the users with enriched multimedia services.
IMS is IP-based telecommunication network architecture with no relationship with the access technology; it can provide services for mobile cellular networks such as Global System for Mobile communications (GSM) and Universal Mobile Telecommunications System (UMTS), besides for Packet Switched (PS) access networks such as General Packet Radio Service (GPRS), Wireless Local Area Network (WLAN), and so on.
The voice communication in the mobile cellular networks such as the GSM and UMTS applies circuit switched technology, and it is called as Circuit Switched (CS) domain which can provide the users the basic voice services and the supplementary services based on voice service. When the CS domain accesses to the IMS, the 3GPP SA2 workgroup proposed IMS Centralized Service (ICS) in order to make the CS users use the uniform IMS service interface, guarantee the consistency and continuity of the services and provide the users with consistent user experience.
IMS centralized services have the following advantages:
(1) The IMS provides service consistency in circuit switched domain and in packet switched domain and supports network integration;
(2) It supports the evolvement from the CS network to the IMS network.
FIG. 1 is an illustration of the application scenarios of the IMS centralized services. As shown in FIG. 1, the ICS user equipment (101) can access to the IP multimedia core network subsystem (104) in the CS domain or PS domain. When both the CS access (102) and the PS access (103) are available and the PS access does not support the bi-directional voice media communication capability, the ICS user equipment should establish CS bearer path and bearer control path via the CS access, and establish the CS session control path via the CS access or the PS session control path via the PS access, and access to the IP multimedia subsystem to implement IMS centralized services.
FIG. 2 is an illustration of the reference architecture of the system of the IMS centralized services. As shown in FIG. 2, the IMS centralized services mainly relate to the following network element equipments:                201: User equipment, UE for short;        202: Visited Mobile Switch Center, VMSC for short;        203: Home Subscriber Server, HSS for short;        204: Media Gateway Control Function, MGCF for short;        205: Media Gateway, MGW for short;        206: IMS CS Control Function, ICCF for short;        207: Call Session Control Function, CSCF for short.        
As shown in FIG. 2, the ICS UE should establish several logical paths to ICCF to access to the IMS: session control path, bearer control path and bearer path.
Wherein, there are the following two bearing ways for the session control path:
(1) Bearing in the CS domain, apply unstructured supplementary service data (USSD) to establish CS session control path through the VMSC and the HSS;
(2) Bearing in the PS domain, apply the Session Initial Protocol (SIP) to establish the PS session control path through the CSCF in the IMS domain and the IP bearer network.
The bearer control path: the UE applies the standard CS control signaling to access to the VMSC, accesses to the IMS through the MGCF, and establishes a bearer control path to the ICCF through the CSCF.
The bearer path: the UE accesses to the IMS through the VMSC and the MGW and establishes a bearer path to establish media connection with the remote UE in the session.
The IMS centralized service uses the session control path to exchange the session control message between the UE and the ICCF, and establish and control the media bearer through the bearer control path, the ICCF acts as the IMS User Agent (UA) to replace the UE to access to the IMS.
For the detailed information of the IMS centralized services, please refer to 3GPP TR23.892: “IP Multimedia System centralized services”.
According to different session control path bearing ways, the implementations of the IMS centralized services are divided into the CS control (Il-cs) and the PS control (Il-ps).
The CS and PS control s have their own advantages and disadvantages. Specifically, the CS session control path uses the USSD transfer mechanism existing in the CS domain, thus it has low requirements for the UE and the network, and when the CS network supports the USSD, the CS control can be applied, however, the USSD message can only transfer 160 byte data at most for one time, and the transmission bandwidth and the service capability are limited; In contrast, the PS session control path uses the PS bearer and has relatively high requirements for the UE and the network, and it requires the user equipment to support the CS and the PS at the same time, however, it has relatively greater bandwidth and stronger service control capability.
In the ICS session based on the PS session control path, when there is PS resource congestion or the CS domain control needs to be used according to the service requirements, the session control path needs to be switched to the CS domain, that is, the CS session control path is applied to control the ICS session; similarly, in the ICS session based on the CS session control path, when the PS control needs to be used according to the service requirements, the session control path needs to be switched to the PS domain, that is, the PS session control path is applied to control the ICS session.
In the prior art, when switching the above session control path, generally the related session is terminated and the corresponding bearer path and bearer control path is released, however, in this way the user call will be affected and the user satisfaction will be degraded.